Hi, I would like to know if there's any alternatives that can replace break-beam sensors in this situation — a gate that counts how many people has passed through using a sensor implemented on one side.
If there aren't any alternatives, can an ultrasonic sensor be used to replace the break-beam sensor? e.g. When a person passed through the gate, the ultrasonic sensor detects a pulse of nearby presence, which the signal is feed into the microcontroller and adds 1.

 

You could put a mirror on one sode and the sensors on the other side.

 

What is causing the constraint of mounting the sensors? As @AlbertHall points out, standard retroreflective sensors can use passive reflectors on the far side, usually a retroreflector like an automotive reflector.

Optoelectronic proximity sensors use the light reflected from the object, but they have particular ranges because the optics have to be angled appropriately.